1. Field of the Invention
The present invention relates to a navigation device mounted on a mobile body for executing route guidance, and in particular to a navigation device which can obtain route data from an outside information center.
2. Description of the Related Art
A navigation device is known for executing route guidance in a vehicle. The navigation device includes a current position detecting section such as a GPS device and a map database. The navigation device recognizes the current position of the vehicle on a map and displays this information on a display. The navigation device also searches, upon inputting a destination, the best route from the current position to the destination. Moreover, if the route is set while the vehicle is traveling, the navigation device gives guidance such as turning right or left at an intersection, in addition to displaying the route on the display.
Some of the navigation devices communicate with an outside information center to obtain information such as congestion information. Furthermore, a system is being proposed in which the navigation device communicate with an outside information center to request a route search and obtains the search result.
By using the outside information center as described above, the navigation device can execute route guidance based on more precise road information.
In the navigation device, the current position detected by the current position detecting device such as the GPS device and the data for road positions in the map data are not particularly precise. Because of this, when the detected current position is marked on a map display based on the map data, there are cases where the display is not accurate.
The vehicle, on the other hand, usually travels on a road. This allows for a process called map matching, in which the trajectory of the current positions is compared with the map data to compensate the position of the vehicle on the map. In this manner, the current position can be moved onto a road and can be accurately displayed.
As described above, by requesting a route search to the information center, a more precise route search result can be obtained. However, in order to communicate with the information center, lines such as a telephone line must be used, and thus, the operation involves some communication cost. Moreover, there are cases where the information center applies fees to its service. Therefore, a cost is incurred for requesting a route search to the information center. In addition, because the mobile body communication usually uses wireless communication, there are cases where sufficient communication cannot be achieved due to the receiving environment for the electric wave.
The map matching is performed with an assumption that the map data is correct. However, road construction is continuously being performed, and when the map data becomes old, the vehicle may travel on a road that is not present in the map data. In such a case, the map matching cannot be correctly performed, resulting in failure to detect the current position.
One of the objects of the present invention is to provide a navigation device which takes advantage of a route search by an information center and which can improve the precision of a map matching process.
According to one aspect of the present invention, there is provided a navigation device mounted on a mobile body for executing route guidance, the navigation device comprising a mobile body route searching device for conducting route search using a map database maintained at the device to obtain route guiding data, an information center route searching device for obtaining route guiding data from an information center using wireless communication, and a search selecting device for determining whether to use the mobile body route searching device or the information center route searching device. In this manner, according to the present invention, a search selecting device is included for allowing selection of the searching device. A suitable searching device can be selected according to various conditions to obtain an accurate route search result while maintaining a suitable communications cost and responsiveness.
It is preferable that the search selecting device selects whether to use the mobile body route searching device or the information center route searching device based on the condition of the optimal route at the route search. For search selection, there are many conditions such as whether search should be conducted with higher priority on highways or on the distance. There are cases where the decision as to whether the mobile body or the information center is more suitable for search can be determined from particular conditions. By considering these conditions for route search, a suitable search device can be selected.
It is also preferable that the route is searched on a plurality of conditions for an optimal route and the search selecting device determines the route searching device for each of the plurality of conditions. By searching on a plurality of conditions, the user can be notified of the optimal route for each condition, and can select the route set from these conditions. By determining the searching device considering factors such as the search load, all the search result can be quickly obtained.
It is also preferable that the route search includes at least a peripheral route search for returning to a set route when off-route, in addition to a complete route search for searching routes to the destination, and the search selecting device determines the route selecting device based on the type of search. The peripheral search may be conducted many times in the case of going off-route or the like. If the search is requested to the information center for each peripheral search, it is likely that the cost will be increased. Moreover it is usually true that the peripheral search needs a quick result, and thus, search at the mobile body is desirable because search can be quickly conducted without spending time to establish a line. Therefore, it is preferable to determine the searching device based on the type of search.
It is also preferable that the search selecting device determines the route selecting device to be used based on the version of the map data in the map database maintained at the navigation device. If the version at the information center is more recent than the version at the mobile body, it is unlikely that the map database at the mobile body will hold accurate road data. Because of this, it is preferable to decide whether or not to request search to the information center considering the version.
It is also preferable that the search selecting device determines a route searching device to be used based on the date when the map data is generated, in the map database maintained at the navigation device. If the date of generating the map is not recent, reliability of the map is decreased and it is thus preferable to select a searching device considering the date when the data was generated.
It is also preferable that the search selecting device divides the route search into two portions, one with the distance to the destination large and the other with the distance to the destination small, and selects the route searching device. It is usually the case that a search result for roads near the current position is needed quickly and that the device should not establish communication. It is also true that the congestion information etc., near the current position can be sufficiently obtained at the mobile body. Therefore, the search for the roads near the current position can often be performed by the mobile body. By considering this point, a suitable searching device can be selected.
It is also preferable that the route selecting device decides the route searching device to be used using operation schedule information of the information center. When the information center is not operated, a search cannot be requested. Therefore, by not communicating in such a case, ineffective communication can be avoided.
It is also preferable that the search selecting device decides route searching location based on the receiving conditions for an electric wave. When the receiving condition for the electric wave is not good, it is probable that the communication with the information center cannot be performed well and there are cases where the search result cannot be obtained. Such ineffective processing can be avoided.
It is also preferable that the search selecting device predicts a time period where the communication can be performed, based on the information on the areas where communication with the information center is possible and the travel condition of the mobile body, and decides a route searching device to be used considering the predicted time period. Even when the navigation device can currently communicate, there are cases where the vehicle runs out of the area where communication is possible before obtaining the search result. Thus, such a possibility can be predicted, and when the probability is high for the vehicle to run out of the area, execution of ineffective processing can be avoided by not requesting a search to the information center.
It is also preferable that the route selecting device decides a route searching device to be used considering the usage condition of the communication with the information center. When the communication section is used for other communications, there are cases where communication with the information center is not possible. By not trying to communicate in such a case, generation of delay by ineffective processing can be avoided.
It is also preferable that the search selecting device decides route searching device considering the past movement history of the mobile body. For example, a user usually has sufficient knowledge on a road if the user has used that road a number of times in the past, and in such a case, it is usually sufficient to use a route search result from the mobile body. By considering the past history, a suitable route searching device can be selected.
It is also preferable that the navigation device further includes a point memory for pre-storing destination candidates and that the search selecting device selects a route searching device considering the destination candidates stored in the point memory. A point stored in the point memory is usually a point the user knows well. Thus, it is usually the case that route search at the mobile body is sufficient for the areas around that point. Therefore, by considering the content in the point memory, a suitable searching device can be selected.
It is also preferable that the search selecting device selects a route searching device considering the communications cost in a predetermined period in the past. Some users wish to set an upper limit to the communications cost. It is thus preferable that an upper limit to the communications cost is set and the selecting device decides whether to request a search to the information center based on a condition indicating whether the upper limit is exceeded. It is preferable that the communications cost includes various service fees at the information center.
It is also preferable that the navigation device includes a display device for displaying the search result, and that the search selecting device selects both mobile body and information center route searching devices to overlap display of the search results from both searching devices.
According to another aspect of the present invention, there is provided a navigation device mounted on a mobile body for executing route guidance, the navigation device comprising a position detecting device for detecting the position of the mobile body, a center route data obtaining device for obtaining route data from an information center using wireless communication, and a map matching device for comparing the detected position of the mobile body with the route data obtained from the information center to compensate the position of the mobile body. The map data at the information center is usually updated to the most recent one. The map data at the mobile body, on the other hand, is not updated to the most recent one. Because of this, there are cases where a new road is included in the route data provided from the information center. In the current position detection at the mobile body, while on the other hand, map matching is performed to draw the current position onto a road. Therefore, if the mobile body obtains a route which includes a new road from the information center and travels along the route, there can be cases where the mobile body travels on a road which is not present in the map data at the mobile body. In such a case, the navigation device will judged as off-route, and no accurate correction to the current position can be obtained. By using the route data from the information center for the map marching, off-route can be prevented and suitable map matching process can be performed.
It is preferable that the map matching device adds route data from the information center as a map matching candidate for map matching at the map data maintained by the mobile body and performs the map matching process. By adding the route data from the information center as a candidate, map matching can be performed without a problem even when the mobile body travels off the route.
It is also preferable that the map matching device compares the route data obtained from the information center with the map data maintained at the mobile body terminal, and when the route data overlap on a road, the map matching device uses only the map data maintained at the mobile body terminal to perform map matching. By not adding a candidate for the overlapped section, processing load can be reduced.
It is also preferable that the map matching device performs map matching using the map data maintained at the mobile body for normal situations, and performs map matching by adding guide route data obtained from the information center as a matching candidate when the detected position of the mobile body is off the roads in the map data. By adding the route data as a candidate only when the mobile body is off-route, the route data can be considered only when necessary, allowing for execution of a suitable map matching process.
It is also preferable that the map matching device considers the version of the map data maintained at the mobile body terminal when judging whether or not to include the route data obtained from the information center as a map matching candidate. When the version is old, it is unlikely that new road data are included, and thus, by considering this, a map matching candidate can be suitably added.
It is also preferable that the map matching device considers the date when the map data maintained at the mobile body terminal is generated, when judging whether or not to add the route data obtained from the information center as a map matching candidate. When the generation date is old, it is unlikely that new road data are included, and thus, by considering this, a map matching candidate can be suitably added.
It is also preferable that the map matching device decides whether or not to add the route data obtained from the information center as a candidate based on the past travel history on the sections where the route data and map data maintained at the mobile body terminal do not overlap.
It is also preferable that the map matching device decides whether or not to add the route data as a map matching candidate considering the travel history for cases when route data is obtained. From the history of the user on whether or not they travel along the route, it can be judged whether or not the route data from the information center should be added as a map matching candidate. For example, for users who do not follow the route, adding a route as a candidate is less important.
It is also preferable that the map matching device changes weights for the roads on the map data and for the route data when executing map matching with the route data added as a map matching candidate. By changing the weights, a suitable map matching can be constantly executed. For example, assigning zero as the weight would mean that the route which was once added is substantially removed from the map matching candidates.
According to another aspect of the present invention, there is provided a route guiding method wherein a route is guided using the route guiding data obtained from a route search using a map database maintained at a mobile body and/or a route guiding data obtained from an information center using wireless communication.
According to another aspect of the present invention, there is provided a method for compensating a position of a mobile body comprising the step of comparing a position of the mobile body detected at the mobile body and route data obtained from an information center using wireless communication.